Default Radioactivity integration

Let's define a default Radioactivity formula to identify content that's interesting both sitewide and within a particular group. I expect that we'll tweak the default formula somewhat, but let's start with a half-life that strongly fades after 24 hours.

Ideally this formula would take into account:

- # of comments
- Likes (ideally, without making the "Like" widget a dependency (#1710714: Theme the Commons "Like" widget)).
- Node views by an authenticated user

Let's also provide some default views as blocks and/or Views content panes. From #1704304: [meta] Pages and corresponding blocks:

- Most active groups
- Most active content in a group
- Active content sitewide
- Active content in your groups
- Active users